The present invention relates to a color picture tube and, more particularly, to a shadow mask thereof and to a method for manufacturing the same.
In a conventional color picture tube as shown in FIG. 1, three electron beams 1 to 3 from electron guns (not shown) are correctly radiated onto red, green and blue phosphors 7 to 9 coated on the inner surface of a panel 6 through apertures 5 regularly formed in a shadow mask 4. The phosphors 7 to 9 then emit red, green and blue light to form a color image.
A shadow mask in a color picture tube of this type must satisfy certain specific requirements. That is, small apertures must be correctly formed in a regular pattern. The shadow mask must be curved in a predetermined radius of curvature. The distance (to be referred to as the g value hereinafter) between the shadow mask and the inner surface of the panel must be maintained at a predetermined value.
When the color picture tube is operated, the electron beams which pass through the apertures formed in the shadow mask comprise 1/3 or less of the electron beams originally emitted by the electron guns. The remaining electron beams bombard against the shadow mask which is, in some cases, thereby heated to a temperature of up to 80.degree. C. As a result, the shadow mask thermally expands to have a g value different from the predetermined g value, thus causing the dome phenomenon. When the dome phenomenon occurs, the color purity of the color picture tube is degraded. The material which is conventionally used for a shadow mask and which contains pure iron as a major component, such as Al-killed decarbonized steel, has a coefficient of thermal expansion of about 12.times.10.sup.-6 /deg. at 0.degree. to 100.degree. C. This material is thus easily vulnerable to the dome phenomenon.
In view of this problem, Japanese Patent Publication No. 42-25446, Japanese Patent Disclosure No. 50-58977 and Japanese Patent Disclosure No. 50-68650 propose the use of a material which has a small coefficient of thermal expansion, such as an iron-nickel alloy, as the material of a shadow mask. However, this proposal has not yet led to a practical use of such a material in a shadow mask. One of the reasons which prevents the use of such a material is the difficulty in working a metal sheet consisting of an iron-nickel alloy. In order that the g value fall within a predetermined allowable range, the curved surface of the shadow mask must be controlled with high precision. The allowable error in a radius of curvature R of 1,000 mm is as small as .+-.5 mm.
An iron-nickel type alloy has an extremely high modulus of elasticity and a high tensile strength after annealing as compared to conventional alloys containing iron as a major component. Accordingly, the iron-nickel type alloy has an inferior curved surface formability by pressing or the like. For example, when a local collapse is formed upon curving an iron-nickel sheet of 0.2 mm thickness to the radius of curvature R as shown in FIG. 2, the degradation in the color purity of the color picture tube is considered negligible if the collapsing quantity d remains 20 .mu.m or less. FIG. 3 shows the collapsing quantity d vs yield point strength characteristics of the material of a 14" type shadow mask. It is seen from the graph shown in FIG. 3, that the yield point strength must be suppressed to 20 kg/mm.sup.2 or less in order to maintain the collapsing quantity d at 20 .mu.m or less. However, a shadow mask consisting of an iron-nickel type alloy has a yield point strength (curve b) as shown in FIG. 4, which is significantly higher than that (curve a) of a shadow mask consisting of a conventional Al-killed decarbonized steel in the case where both are annealed in hydrogen in an annealing furnace generally used for the conventional Al-killed decarbonized steel. Even if a shadow mask consisting of an iron-nickel type alloy is annealed at a high temperature of 900.degree. C., the yield point strength is only lowered to 29 to 30 kg/mm.sup.2. Referring to FIG. 3, since no clear boundary was obtained for the yield point strength of the iron-nickel type alloy, a tensile strength after 0.2% elongation is plotted instead. Since a shadow mask consisting of an iron-nickel type alloy has a small coefficient of thermal expansion, degradation in color purity due to a high coefficient of thermal expansion is substantially eliminated. However, degradation in color purity due to deformation and a large collapsing quantity upon curved surface formation still remains.